Study Finds Falling Trees Make Sound Even if No One is Around to Hear It

It's a riddle that's been twisting brains for generations: If a tree falls in the woods and no one is around to hear it, does it make a sound? The answer, at least according to a comprehensive study of nature's noises, is 'yes' -- but it may be drowned out by the hum of a motor vehicle. For the last two years, researchers have been placing microphones throughout Oregon's Crater Lake National Park in order to database and analyze what sort of sounds can be heard in some of the remotest places. And it turns out, evidence of human activity is far more pervasive than you might have imagined.Since researcher Scott McFarland began setting up microphones across the national park and listening back to the sounds they captured, he's honed a particularly acute sense of hearing. Most of the noises his 20 listening stations have picked up are things you might imagine -- the rustle of leaves, crack of falling branches, or the noshing of some grazing critter -- but it doesn't end there.

"Anything you could possibly think of hearing, we probably have a recording of it," McFarland tells Oregon Public Broadcasting. "Everything from badgers and porcupines grunting to the wings of a mosquito flying by."

Unfortunately, however, even in the most remote locales in the United States, regions set aside for preservation, the presence of human activity almost never ceases to be audible. McFarland says that, as he plays back his nature recordings for databasing purposes, the sounds of air traffic can be heard about 20 percent of the time -- and that's in addition to the cars and other motor vehicles buzzing in the distance. Silence, or rather the lack of human noise, is hard to come by these days.

In fact, airplane flyover noise is present about 30 percent of the time in virtually every part of the continental United States. McFarland says the sound of airplanes has become so constant, most people don't even hear it.

"You're acclimated to the noise. It's amazing how much we've lost the ability to hear sounds."

Researchers say that while such human-caused noises may blend into the background for us, the nearly constant drone of engines could be hurting wildlife. Evidence of the negative impact of noise pollution is well-documented in marine habitats, though increased levels of stress have been recorded in land animals as well.

"Think about how quiet a mouse would be under the snow. Owls have the ability to hear that," McFarland tells NPR. "So just a small increase in noise can really limit the area that they can actually hunt."

Once the study is completed, the National Parks Service plans on using the sound database to determine whether or not to allow a proposed helicopter tour company to operate over Crater Lake.